The present invention includes a method for improving heat transfer and reducing maintenance-related down time in a circulating water system. The invention also encompasses an improved sump pump apparatus useful in conjunction with the method of the invention. More particularly, the method of the invention reduces plugging in cooling water heat exchangers in a closed-loop circulating cooling water system.
In closed loop liquid circulation circuits, such as cooling tower systems, particulates tend to accumulate in the water reservoir or sump. In systems closed to the atmosphere, the particulates include corrosion products, precipitated mineral salts and precipitates formed by chemical additives. Systems open to the atmosphere, on the other hand, accumulate not only corrosion products and precipitated minerals but also airborne dirt, stones and vegetation.
For a general discussion of circulating water systems and particularly the associated corrosion treatment programs, see the Nalco Water Handbook, McGraw-Hill, 1979. The following background information is drawn from 24 Kirk Othmer Encyclopaedia of Chemical Technology, 3rd Ed. 385.
Improvements in the field of circulating water systems, particularly cooling water systems would indeed contribute to reducing maintenance and energy costs in industrial facilities. Cooling is the largest industrial use for water. Generally, cooling water quality should be maintained to neither permit the formation of deposits, which reduce heat transfer or increase resistance to the flow of water, nor permit significant deterioration of the materials of construction in the cooling system.
Accumulation of deposits in process pumps and heat exchange equipment can reduce heat transfer, reduce water flow, cause premature equipment deterioration or failure and reduce product quality or yield. Unscheduled shutdowns to repair pump failures or plugged heat exchangers cost valuable production time.
Deposits may consist of sedimentary materials, e.g. sand, silt, or clay; corrosion products, e.g. rust, formed in place or transported from another part of the system; crystalline scales formed in situ; bacterial or fungal slimes; or other materials. Usually, water system deposit consist of mixtures of several types of materials. Reduced water flow results from the same types of deposits but usually in larger accumulations and at locations other than heat-transfer surfaces. Premature equipment deterioration or failure may be caused by contact with the waters used in industrial systems. Corrosion of metallic equipment and components is the most common manifestation, but non-metallic materials, e.g. wood, concrete and plastics, also deteriorate as a result of contact with industrial waters. Equipment deterioration may also be caused by erosion or abrasion by solids suspended in the water; equipment may fail catastrophically as a result of water-caused problems. Reduction of product quality can be caused by impurities present in the water entering a system or by those which develop as a result of corrosion, bacterial growth, or other problems within the system.
Both the composition of make-up water as well as the recirculation rate affect the development of waterside problems including the accumulation of particulates. In once-through systems drawing water from rivers,lakes, or the sea and returning it directly thereto after passage through heat-exchange equipment, the water problems depend upon the composition of the influent water since there is little, if any, change in water composition during its passage through the process equipment.
Nonevaporative, closed circulating systems include hot-water heating, high temperature water, chilled water, combined chilled and hot water, solar heating, snow melting, and brine systems. These systems frequently involve a cyclic change in temperature during operation and minor changes in composition develop primarily as a result of a reaction of the water with system components. In theory, such systems require no make-up water because they are closed and, therefore, water-caused problems should be minimal. However, the constant influx of make-up water together with essentially no flushing, or blowdown, leads to accumulation of particulates generated by oxidation, scale formation, and bacterial or fungal growth.
Evaporative, open circulating systems include spray-type humidifiers and cooling water-systems, e.g. cooling towers, evaporative condensers, or evaporative coolers. The evaporation increases the dissolved solids, leading to scale formation. In addition, gases and solids are scrubbed from the air through which the water is sprayed. Oxygen and sometimes sulfur oxides from stack gases are dissolved, increasing the corrosivity of the water. Particulates scrubbed from the air abrade moving parts, form heat-insulating deposits, promote localized corrosion, or result in undesirable growths of microorganisms.
In both open and closed circulating water systems, temperature increases promote scale formation and corrosion. Dissolved-solids concentrations increase in evaporative systems and promote scale formation. Condensation and dilution promote corrosion which produces particulate corrosion products. Any contact with air further accelerates corrosion by oxygenating the water, but spraying of the water through air as in an evaporative cooling tower adds not only oxygen but also finely divided solids and microorganisms to the water. While circulating water systems behave differently depending on the relative severities of these various factors, all of the factors contribute to some extent to cause operational problems resulting from the circulation and deposition of particulate solids within the system.
Suspended or colloidal solids can be reduced or removed by rough screening, sedimentation, centriugal separation, strainging, filtration, coagulation, flocculation, magnetic separation, or combinations of these processes. Clarification frequently describes a combination of these processes. The selection of the appropriate method or methods for each case depends upon the maximum concentration acceptable for the final use of the water. Particle size is an important consideration for the use of screening, straining, filtration, and coagulation.
Rough screening is used for the removal of large objects, e.g. logs, fish, masses of water weeds or algae, and other floating debris. Depending upon the screen opening, it may be used for the removal of particles as small as about 5 mm. Equipment for rough screening ranges from trash rack with bars spaced several inches apart, commonly called grizzlies, to screens with openings as small as 5 mm.
Straining and filtration are used for the removal of all types of particulate matter as small as about 0.01 mm. Woven mesh is available in a broad range of openings and a variety of weaves and materials. Granular media for filtration include sand and gravel, anthracite coal, and garnet. Mixed granular-media filters provide greater in-depth loading and, therefore, longer runs between backwashing than do single-media filters. They can be used for waters containing up to about 500 mg/L or less of suspended matter, whereas woven-media filters or strainers are practical for waters with about one tenth this concentration. Woven-media filters precoated with filter aids, e.g. diatomaceous earth, are used to polish waters containing about 10 mg/L or less of suspended solids, and remove particles as small as 0.001 mm. Membrane filters are used for ultrafiltration to remove even smaller particles.
The pump which withdraws water from the reservoir or sump is typically equipped with a single stage coarse strainer to prevent the largest of these particles from entering and damaging the pump and downstream equipment.
Smaller particles are segregated from the circulating water stream by in-line filters. One of the more popular in-line filters comprises an elongated woven conical screen fixed to an annular ring drilled with bolt holes. The conical screen is held in place by positioning the annular ring between the faces of a bolted flanged pipe fitting. While this design is relatively simple and inexpensive, it unfortunately requires flanged connections to be broken to clean out the filter screens. This maintenance typically requires a system shut-down.
U.S. Pat. No. 1,909,578 to Franke teaches a floating pump which includes a mesh filter to prevent entry of grit into the pump. U.S. Pat. No. 4,461,614 to Niedermeyer discloses a sump pump having a debris trap to prevent small debris such as sand and stones from being drawn into the pump by the inpeller. The strainer may optionally be hinged so that unfiltered liquid may flow directly into the impeller assembly.
Previous developments in the field of circulating water systems have, however, failed to provide for a convenient method for decreasing particulate accumulation in the associated downstream process equipment and to provide for orderly shutdown of the circulating water circuit when maintenance cleaning is required.